fix(codersdk/toolsdk): fix tool schemata (#17365)

Fixes two issues with the MCP server:
- Ensures we have a non-null schema, as the following schema was making
claude-code unhappy:

 
```
        "inputSchema": { "type": "object", "properties": null },
```


- Skip adding the coder_report_task tool if an agent client is not
available. Otherwise the agent may try to report tasks and get confused.

codersdk/toolsdk/toolsdk.go

@@ -259,6 +259,10 @@ is provisioned correctly and the agent can connect to the control plane.
 		Tool: aisdk.Tool{
 			Name:        "coder_list_templates",
 			Description: "Lists templates for the authenticated user.",
+			Schema: aisdk.Schema{
+				Properties: map[string]any{},
+				Required:   []string{},
+			},
 		},
 		Handler: func(ctx context.Context, _ map[string]any) ([]MinimalTemplate, error) {
 			client, err := clientFromContext(ctx)
@@ -318,6 +322,10 @@ is provisioned correctly and the agent can connect to the control plane.
 		Tool: aisdk.Tool{
 			Name:        "coder_get_authenticated_user",
 			Description: "Get the currently authenticated user, similar to the `whoami` command.",
+			Schema: aisdk.Schema{
+				Properties: map[string]any{},
+				Required:   []string{},
+			},
 		},
 		Handler: func(ctx context.Context, _ map[string]any) (codersdk.User, error) {
 			client, err := clientFromContext(ctx)